Computer-controlled automatic plotting machines have come into wide application in recent years. The writing instrument used in these machines is known as a plotter pen. The plotter pen is mounted, either singularly or in combination with other plotter pens if multi-colored drawings are required, on the plotting machine at a location where the writing tip of the pen can come into contact with paper or other writing surface. Each plotter pen contains a cartridge having a discrete supply of writing fluid
During the course of a computer-controlled creation of a drawing, it is possible that a plotter pen cartridge may run out of writing fluid. If the operator is not aware of the depletion of writing fluid, the plotting machine will continue to operate but a drawing will not be created. Should this result occur, both time is wasted and additional expense is incurred in having to redo the drawing.
Plotter pens are also used in other applications such as chart recorders placed on oil and gas wells or used in electric power and chemical plants. A depletion of the supply of writing fluid would result in the loss of critical data necessary for the operation of the well or the plant.
In order to minimize the possibility of the writing fluid supply being depleted in the middle of a project, operators have been replacing the plotter pen while it still has a usable quantity of writing fluid left inside. This is not cost efficient and also increases the amount of time that the plotting machine or chart recorder is out of service for maintenance
DESCRIPTION OF THE PRIOR ART
It is known to provide motor-driven fountain pens with a magnetic sensor that triggers a micro-switch to disengage the driving motor when the ink supply becomes depleted; e.g. U.S. Pat. No. 4,350,458 to Murahara et al. Magnets used in combination with reed-switches or microswitches to sense either fluid level or fluid flow are also common. For example, U.S. Pat. No. 4,290,059 to Noyes discloses using a magnetic sensing device that triggers a visual alarm when the level of an oil storage tank decreases to a certain point. Complementarily, a similar detector can be used to sense when a storage container has been filled to a predetermined level as in U.S. Pat. No. 3,389,603 to Jacobs. Magnetic sensor devices used in a fluid flow environment are disclosed in U.S. Pat. No. 4,615,303 to Sackett.
It is also known to provide computer-controlled plotter pens with a transparent ink reservoir so that an operator may visually determine how much ink is remaining. However, this approach requires constant monitoring by the operator to both notice when the plotter pen begins to run low on ink and then replace the ink supply before the pen runs out and the drawing is ruined.
Many expensive plotters keep track of the plotting time. The plotter operator attempts to determine a minimum plot time and then remove the pen before the ink supply runs out. This approach does not always produce good results because of variations that cannot be anticipated by the operator The volume of the ink supply contained in a plotter pen can vary from pen to pen during the manufacture of each pen. The quantity of ink laid down varies depending on the type of pen point used, as well as on the type of material used as the writing surface. The speed and complexity of the plotting affects the amount of ink used; long straight lines plot faster and use up more ink than do short lines or lettering. In order to have a safety margin, operators may throw away pens that still have one-third to one-half of their ink supply remaining.
SUMMARY OF THE INVENTION
The object of this invention is to overcome the drawbacks of the prior art by providing a plotter pen assembly with magnetic sensing means that alerts the operator when the ink supply has reached a predetermined level. Another object is to utilize the magnetic sensing means in combination with a control device that allows unattended operation of a computer-controlled automatic plotter by providing for automatically discontinuing the operation of the plotter when the ink supply runs out thereby preventing the drawing having to completely be redone.
The magnetic sensing means of the present invention is carried by an ink-following object that follows the column of ink inside the cartridge of the plotter pen assembly. As the ink supply is depleted, the ink-following object passes adjacent to a magnet sensing device. When the magnet sensing device senses the magnet carried by the ink-following object, a signal is sent to an alarm to either visually or audibly alert the operator that the ink supply has reached a predetermined level. Alternatively, the magnet sensing device sends a signal to a control device that automatically shuts down the plotter to permit replacement of the ink supply. A new plotter pen assembly having a full ink supply is then manually or automatically put into service. Once the ink supply is replaced, the plotter can be restarted at the point it was stopped and the drawing can be completed.